Cascade fuel inlet manifold for fuel cells

ABSTRACT

Fuel is provided to an inlet ( 14 ) of a cascade region ( 15 ) which has a plurality of stages ( 17 - 23 ), each of which divides fuel flow evenly into a pair of corresponding slots ( 24 - 26 ). The flow is then spread across a floor surface ( 41 ) of a cascade exit header ( 40 ), the flow spreading into areas between the slots. The flow is then directed into an open cavity which is in fluid communication with the inlets of the fuel flow fields ( 12 ) of the fuel cells, reaching the fuel flow field inlets uniformly and simultaneously.

TECHNICAL FIELD

[0001] This invention relates to a fuel inlet manifold for fuel cells inwhich the fuel flow is evenly split a consecutive number of times, in acascade fashion, and spread across the face of the fuel flow fieldinlets of the fuel cell stack, thereby to ensure uniform delivery offuel simultaneously to all of the individual fuel cell fuel flow fieldsduring startup or transient operation of a fuel cell stack.

BACKGROUND ART

[0002] Prior methods for creating flow uniformity consist primarily ofdiffusers that expand the flow over a large area or restrictive devicessuch as screens or orifice plates. The former incur a volume penaltybecause a gradual expansion is required to avoid flow mal-distributionfrom separation and “jetting” of the core flow. The latter force a flowredistribution across the exit plane and can be quite compact, but anunacceptable pressure loss is often required to create the necessaryuniformity. Unacceptable mal-distributions of flow may also occur fromlocalized “jetting” through the exit plane of the inlet flowdistribution apparatus, requiring impingement plates or deflectors toimprove the uniformity. These types of designs tend to be effective onlyat design point flow velocities and do not perform well over wide rangesof flows. Furthermore, in both diffuser and restrictive type devices,the requirement for simultaneous distribution and delivery of an inletfluid element across the entire inlet manifold exit plane is not metbecause fluid from the inlet pipe crosses the manifold exit plane nearthe center of the flow field first. During transient conditions, e.g.,ramping power up from, say, 50% to near 100% power, if the fuel flowsare non-uniform, some cells will not get enough fuel, resulting in poor(possibly inadequate) fuel cell stack performance.

DISCLOSURE OF INVENTION

[0003] Objects of the invention include: provision of a PEM fuel cellfuel inlet which provides a rate of flow of fuel which is substantiallyuniform to the flow fields of all of the fuel cells in a fuel cellstack, and which delivers a substantially uniform amount of fuelsubstantially simultaneously to each of the flow fields in a fuel cellstack; substantially simultaneous provision of substantially equalamounts of fuel to all of the fuel flow fields of fuel cell stacksduring start up and other transient fuel flow conditions; increasing thedurability of fuel cell stacks; improved start up of fuel cells;improved fuel cell transient response; and improved fuel flowdistribution in fuel cell stacks.

[0004] The invention is partially predicated on the recognition of thefact that although a simple pipe manifold that splits the flow intoseveral, equal-length passages may partially resolve the aforementionedflow problems by delivering fluid to diverse locations across the exitplane, which may be effective in steady-state conditions, such a devicehas a volume penalty and does not resolve the issue of localized“jetting” unless it has an extremely large number of legs to distributethe flow with fine resolution.

[0005] According to the present invention, the fuel inlet flow controlapparatus of a fuel cell evenly divides the fuel flow several times,successively, to provide a number of separate flows, and then spreadsthe flows, so as to distribute the fuel substantially uniformly acrossthe entrances to all of the flow fields in the fuel cell stack, wherebyfuel flow transients approach the fuel flow fields of all of the fuelcells in the stack at substantially the same flow rate and substantiallysimultaneously during start up and other transient fuel flow conditions.In a disclosed embodiment of the invention, a cascade region comprisesseveral levels, the fuel being split and caused to flow in two separatedirections at each level, whereby an initial singular flow results in anumber of flows at cascade outlet passages, such as slots, said number,for instance being eight or sixteen, or any other suitable number. Inthis embodiment, the flow in the cascade outlet slots impinges on a flatsurface which assists in directing the flow from the outlet slotsuniformly across the flat surface (although the surface could be curvedin other applications), and the flow direction is turned toward theinlets of the fuel cell fuel flow fields. An open fuel inlet cavityreceives a uniform flow of fuel which approaches the entire extent ofthe cavity simultaneously, the fuel flow field inlets for each fuel cellbeing in fluid communication with the cavity, whereby changes in fuelflow reach all of the fuel cells simultaneously with substantially auniform flow into all of the fuel cells in the stack.

[0006] The invention, through uniform distribution of fuel, alsoenhances performance during normal operation of a fuel cell stack,especially during fuel flow transients.

[0007] Other objects, features and advantages of the present inventionwill become more apparent in the light of the following detaileddescription of exemplary embodiments thereof, as illustrated in theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a partially sectioned, partially broken away perspectiveview of one embodiment of a cascade fuel inlet manifold according to thepresent invention.

[0009]FIG. 2 is a stylized, idealized schematic diagram of fuel flow,taken approximately on the line 2-2 of FIG. 1.

[0010]FIG. 3 is a simplified, stylized illustration of fuel flow takenapproximately on the line 3-3 in FIG. 1.

MODE(S) FOR CARRYING OUT THE INVENTION

[0011] Referring to FIG. 1, a plurality of fuel cells are arrangedcontiguously within a fuel cell stack 11. Each of the fuel cells has afuel flow field 12 which receives fuel reactant gas and distributes itthroughout each fuel cell. The fuel flow fields 12 illustrated in FIG. 1may represent only a fraction of the fuel flow fields of each fuel cellstack, there may be a turnaround manifold (not shown) to the upper rightof FIG. 1, receiving fuel reactant gas flowing through the flow fields12, and there may be additional fuel flow fields, one for each fuelcell, disposed beneath those illustrated in FIG. 1, the fuel flowingoutwardly therefrom into a fuel exit manifold disposed toward the lowerleft of the illustration of FIG. 1. The nature of the fuel flow fieldsand the arrangement of the fuel exit manifolding is irrelevant to thepresent invention.

[0012] In FIG. 1, fuel is introduced through an inlet 14 of a cascaderegion 15, the fuel being provided to the inlet 14 by a conventionalfuel conduit. The cascade region 15 has a plurality of stages orplateaus 17-23, each of which has an upper surface which terminates in apair of corresponding passages, such as slots 24-26. As shown by thesmall flow arrows in FIG. 1, on each plateau 17-23 of the cascade, theflow splits so that substantially half of it will flow toward each ofthe slots 24-26 at the edge of the corresponding plateau. This causesthe flow to be spread across a cascade exit surface which comprises afloor surface 41 of a cascade exit header 40, which extends throughoutthe area below the cascade region 15 as seen in FIG. 1, and is in fluidcommunication with each of the slots 26.

[0013] The flow, being directed downward toward the floor surface 41 ofthe cascade exit header 40, spreads into areas between the slots, as isillustrated in FIG. 2. In FIG. 2, the flow exiting any one of the slots26 spreads out, flowing to the right and left as seen in FIG. 2 (to theupper left and lower right as seen in FIG. 1), as well as flowingupwardly in FIG. 2 (to the upper right as seen in FIG. 1), toward anopen cavity 46. Referring to FIG. 3, the flow from the cascade exitheader 40 is to the right into the open cavity 46. The flow is evenlydistributed from the upper left to the lower right as seen in FIG. 1, asthe flow enters the open cavity 46 (FIG. 3). The flow will then enterthe lower channels of the flow fields 12 as well as flowing upwardly inthe open cavity 46 so as to enter upper channels of the flow fields 12.As an initial flow of fuel is passed through the cascade region, thecascade exit header and the open cavity 46, into the flow fields 12, itmay form a fuel/air (or fuel/inert gas) interface, somewhat as shown bythe dot/dash line 49 in FIG. 3. The fact that each fuel cell does nothave the same degree of fuel penetration in all areas of its own flowfield is irrelevant; what is important is that all of the fuel cellshave the same degree of penetration (such as illustrated by the dot dashinterface line 49) at any point in time, and that such degree of fuelpenetration occurs simultaneously in all fuel cells.

[0014] In FIG. 1, the initial flow distribution within the cascaderegion 15 splits the flow a number of times, with each path being equalin length and geometry so that the flow rates and pressure drops aresubstantially the same, and the arrival time of the fuel/air interfaceduring startup, or change in fuel flow rate during transient conditions,is substantially simultaneous at each of the cascade outlet slots 26. Inthe orientation of the cascade region 15 shown in FIG. 1, liquid waterin the fuel flow is not able to collect in the passages and createnon-uniformities in the flow cross section, and therefore in the flowdistribution. However, if desired in certain utilizations of the presentinvention, the cascade region 15 could be oriented in a manner which isnormal to that shown in FIG. 1; this may be visualized by thinking thatthe cascade region 15 is rotated 90° counterclockwise about a pivotpoint 50 in FIG. 3. However, if the exit slots 26 were to flow the fueldirectly into the open cavity 46, then the open cavity 46 would have tobe provided with a baffle (similar to the floor surface 41) or the depthof the cavity 46 would have to be increased significantly in order toget the desired spreading that is provided by the floor surface 41 inthe embodiment shown herein.

[0015] In the example herein, the cascade has three stages; evenlydividing the flow seven times, yielding eight flows. However, two (ormore than three) stages, yielding four (or more than eight) flows may beused if desired. If FIG. 1 only represents a fraction of the inlet sideof the fuel flow fields, the balance being to the upper left of FIG. 1,an initial stage will split the flow so that some fraction of it entersthe inlet 14 (and similarly for the other portion).

[0016] The present invention causes the fuel/air (or fuel/inert gas)interface, and other changes in the fuel flow, to arrive at the inletsof the fuel flow fields of all of the fuel cells simultaneously. Thismeans that the differences between electrical activity within each ofthe fuel cells will be dependent upon the characteristic of theindividual fuel cells, rather than on the fact that one fuel cell hasreceived a greater change in the quantity of fuel than other fuel cells.This in turn allows the electrical potential of all of the cells to bemore uniformly controlled, while at the same time minimizing any damageto the individual fuel cells, decreasing variations of performance,improving transient capability, and significantly increasing fuel celllife. In most applications, the present invention provides sufficientcontrol over relative voltages of the fuel cells during startup so thatan inert gas purging of the fuel flow fields need not be undertaken.Flow impingement on the various plateaus of the cascade improves uniformspreading of the fuel in the dimension parallel to the slots, while atthe same time the cascade is spreading the fuel quite uniformly in thedimension normal to the slots. Impingement of the fuel on the floorsurface 41 of the cascade exit 40 (FIG. 3) spreads the fuel indimensions not parallel with the slots. The cascade and the headerprevent “jetting” of the flow due to sudden expansions. At start up,jetting tends to mix the fuel and the air volumes at the fuel/airinterface, which creates heat and excessive voltages, and which createsa safety hazard due to the combustible nature of the mixture. Duringoperation with a conventional manifold design, transient conditionsresult in flow mal-distribution at the fuel flow inlets. Themal-distribution can cause cell performance degradation. However, withthe manifold of this invention, uniform flow to all the fuel flow fieldinlets is maintained during transient conditions.

[0017] The fuel inlet manifold of the invention may be used with fuelcell stacks other than PEM fuel cell stacks.

[0018] Thus, although the invention has been shown and described withrespect to exemplary embodiments thereof, it should be understood bythose skilled in the art that the foregoing and various other changes,omissions and additions may be made therein and thereto, withoutdeparting from the spirit and scope of the invention.

We claim:
 1. A fuel cell stack having an array of contiguous fuel cells,each fuel cell having a fuel flow field with a flow field inlet, saidstack including a fuel inlet manifold system comprising: a cascaderegion having a series of stages, each stage having a surfaceterminating in passages at opposite edges of said surface, each passageof each stage except the last in said series directing fuel to thesurface of a next stage in said series, the passages of the last stagein said series directing fuel onto a cascade exit surface so that fuelis spread in dimensions not parallel with said passages; and an opencavity disposed between said flow field inlets and said cascade exitsurface.
 2. A fuel cell stack having an array of contiguous fuel cells,each fuel cell having a fuel flow field with a flow field inlet,comprising: means for providing a flow of fuel from a source; means forsuccessively dividing the flow of fuel substantially equally into twosubstantially simultaneous flows, a number, n, of times, to provide n+1flows, said number being at least three; and means for spreading thefuel from said n+1 flows to provide a single, substantially uniform flowof fuel to said flow field inlets.
 3. A method of providing fuel to afuel cell stack having an array of contiguous fuel cells, each fuel cellhaving a fuel flow field with a flow field inlet, said methodcomprising: providing a flow of fuel from a source; successivelydividing the flow of fuel substantially equally into two substantiallysimultaneous flows, a number, n, of times, to provide n+1 flows, saidnumber being at least three; and spreading the fuel from said n+1 flowsto provide a single, substantially uniform flow of fuel to said flowfield inlets.